There is blood in my urine.. when i urinate, i get a burning sensation. yes i've had sexy before. could it be an std?? which could it most likely be??

Many possibilities: Depending on when you saw the blood compared to the time you had sex, it could be an STD. it could also be a kidney stone (or 1 starting), kidney probs, or a urinary infection. Given your age, a tumor or polyp of the bladder isn't likely, but stranger things have happened. In any case, you should get checked out by a Urologist. Saving your urine in a clean jar may provide a sample for tests. GL!